[Haskell-cafe] Code that doesn't compile - but should :)

[Chris Eidhof]

What about this?

> {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
>  UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
>  TypeOperators, TypeSynonymInstances #-}
> 
> data Data k = Pair Integer (() -> k)
> data RecData = RecData (Data RecData)
> mk_data x = RecData(Pair x (\() -> mk_data (x+1)))
> 

The I had to change the type of the Converter typeclass

> class Converter a f where convert :: f a -> a
> 
> -- instance Converter RecData Data where
> --  convert (RecData r) = r
> 
> class Selector s a where select :: s -> a
> 

And explicitly quantify the type variables:

> f :: forall f s a . (Selector s (a->f a), Converter a f) => s -> (a->a)
> f s =
>  let method = select s :: a -> f a
>  in (\x ->
>        let res = method x
>        in convert res)

-chris

On 16 mrt 2010, at 10:36, Giuseppe Maggiore wrote:

> The error message (obtained by loading the file with ghci) is:
> GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main             ( C:\Users\pulcy\Desktop\Papers\Monads\Objec
> tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )
> 
> C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
> Problems.hs:18:15:
>     Could not deduce (Selector s (f a -> a))
>       from the context (Selector s (a1 -> f1 a1), Converter a1 f1)
>       arising from a use of `select'
>                    at C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
> t\Experiments\FunctorsProblems.hs:18:15-22
>     Possible fix:
>       add (Selector s (f a -> a)) to the context of
>         the type signature for `f'
>       or add an instance declaration for (Selector s (f a -> a))
>     In the expression: select s
>     In the definition of `method': method = select s
>     In the expression:
>         let method = select s in (\ x -> let res = ... in convert res)
> 
> C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
> Problems.hs:21:11:
>     Couldn't match expected type `a1' against inferred type `f a'
>       `a1' is a rigid type variable bound by
>            the type signature for `f'
>              at C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
> riments\FunctorsProblems.hs:16:18
>     In the expression: convert res
>     In the expression: let res = method x in convert res
>     In the expression: (\ x -> let res = method x in convert res)
> Failed, modules loaded: none.
> Prelude>
> 
>  
> On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> wrote:
> Giuseppe Maggiore <giuseppemag at gmail.com> writes:
> 
> > Hi! Can anyone tell me why this code does not work? I cannot seem to
> > figure why it is broken...
> 
> The error message (and how you got it) would help...
> 
> > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
> >   UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
> >   TypeOperators, TypeSynonymInstances #-}
> 
> You sure you have enough language extensions there? ;-)
>  
> Barely :)
>  
> 
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com
> 
> 
> 
> -- 
> Giuseppe Maggiore
> Ph.D. Student (Languages and Games)
> Microsoft Student Partner
> Mobile: +393319040031
> Office: +390412348444
> 
> 
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